Circuits that generate a constant output voltage independent of fluctuations in temperature and supply voltage are required in multifarious ways in semiconductor circuit engineering. They are used across the board in analog, digital and mixed analog/digital circuits. A frequently used type of such circuits are the so-called BGR (Bandgap Reference) circuits.
The basic principle of a BGR circuit is to add two partial signals (voltages or currents) that exhibit an opposite temperature characteristic. Whereas one of the two partial signals drops with increasing temperature, the other partial signal rises with increasing temperature. An output voltage that is temperature-constant over a certain range is then derived from the sum of the two partial signals. The output voltage of a BGR circuit is also denoted as reference voltage below in accordance with customary usage.
A stable operating point of a BGR circuit is situated at a Bandgap voltage of 1.211 V. This reference voltage can be converted into yet other voltages by means of a voltage divider. A BGR circuit can have a further stable operating point at 0 V depending on the offset of an operational amplifier used for the BGR circuit and on leakage current. Situated between the two stable operating points is an unstable operating point. This unstable operating point is in the vicinity of 0 V in the case of small leakage currents and small offset voltages. When starting the BGR circuit, the BGR circuit must be brought from the stable operating point at 0 V to the higher stable operating point that is derived from the Bandgap voltage of 1.211 V. An additional start-up circuit is generally used for this purpose.
In order to set the higher operating point in the BGR circuit, an external setting current is frequently fed into the BGR circuit. This setting current must be switched off completely during normal operation of the BGR circuit.
During the introduction of new technologies, which are not stable at high volumes, the unstable operating point can be displaced by several 100 mV toward positive voltages because of impaired offset and leakage current properties. If the switch-off point of the external setting current is subjected to high fluctuations because of a strong dependence on process and matching, the switch-off points must be selected to be so low when developing the BGR circuit that the BGR circuit is not influenced by the setting current during normal operation. However, a low switch-off point can lead to problems in the BGR circuit, since it may be that the unstable operating point is reached instead of the higher stable operating point.
Therefore, when setting the higher stable operating point the starting performance of the BGR circuit is monitored so that the switch-off point of the setting current can be determined as accurately as possible. Two modes of procedure are known for this purpose. Firstly, the output voltage of the BGR circuit can be monitored. Secondly, the current in a BGR cell can be measured.
The determination of the current through the BGR cell has proved to be the better of the two modes of procedure, since the switch-off point can be set to 1/100, 1/10 or ½ of the operating current of the BGR cell. The switch-off point is set to ¼ of the operating current of the BGR cell in order to design as robustly as possible a circuit that serves for setting the operating point of the BGR circuit and for subsequently switching off the setting current.
When connecting a resistive load to the BGR circuit, it is to be ensured that a large portion of the output current flows into the load and not through the BGR cell. Consequently, the output current of the BGR circuit is not suitable in this case for determining the current in the BGR cell.
A BGR circuit with a setting circuit for setting the operating point of the BGR circuit is described in European patent application EP 1 063 578 A1. For this purpose, the reference voltage generated by the BGR circuit is compared with a voltage that is situated in a voltage range between the desired operating point and a metastable operating point. Other BGR circuits with associated setting circuits for setting the operating point of the BGR circuit are to be found in US patents U.S. Pat. No. 5,087,830 A, U.S. Pat. No. 6,346,848 B1 and U.S. Pat. No. 5,867,013 A.
FIG. 1 illustrates a known BGR circuit 1 and setting circuit 2. The BGR circuit 1 has an operational amplifier OP1, resistors R1, R2, R3 and R4, and diodes D1 and D2. Here, resistors R1, R2 and R3 as well as the diodes D1 and D2 are assigned inside the BGR circuit 1 to a BGR cell 3.
The resistors R2 and R1 as well as the diode D2 are arranged serially in the specified sequence. One end of this series circuit is connected to the output of the operational amplifier OP1, and the other end is connected to ground VSS. In the same way, the resistor R3 and the diode D1 are connected in series and connected to the output of the operational amplifier OP1 and to ground VSS.
The connecting line between the resistors R1 and R2 is connected to the inverted input of the operational amplifier OP1. The connecting line between the resistor R3 and the diode D1 is connected to the non-inverted input of the operational amplifier OP1 via a further connecting line. An additional current Iein can be coupled into this further connecting line. A resistor R4 is also connected between the output of the operational amplifier OP1 and ground VSS.
The output of the operational amplifier OP1 also constitutes the output of the BGR circuit 1. A temperature-stabilized reference voltage can be tapped at the output of the BGR circuit 1 during its normal operation. The temperature stability of the reference voltage is based on the opposite nature of the temperature dependencies of the two voltages that drop across the resistor R3 and across the diode D1, respectively. The diode D1 and the diode D2 can be constructed in each case, for example, from a bipolar transistor whose base terminal is connected to its collector terminal. The base/emitter voltage of the diode D1 then has, for example, a temperature coefficient of −2 mV/K. The temperature dependence of the voltage dropping across the resistor R3 is a function of the dimensioning of the resistors R1, R2 and R3, and of the temperature coefficients of the thermal voltage VT of the diode D2. The voltage dropping across the resistor R3 has a temperature coefficient of +2 mV/K, owing to a suitable selection of these components and because of the design of the BGR circuit 1 in terms of circuit engineering. This results overall in a reference voltage that is stable over a certain temperature range.
The setting circuit 2 is connected downstream of the BGR circuit 1. The setting circuit 2 comprises transistors N1, N2, P1, P2, P3 and P4, as well as a constant current source I1. The transistors N1, N2, P1, P2, P3 and P4 are MOSFETs. The respective doping of their channels is specified by the letters N and P respectively, in their reference symbols. This nomenclature also applies to transistors mentioned below.
The transistors N1 and N2 are connected in a current mirror circuit downstream of the input of the setting circuit 2. Flowing in this case through the transistor N1 is the input current of the setting circuit 2, which is at the same time the output current of the BGR circuit 1. The mirrored input current flows through the transistor N2 into the transistor P1, which is connected, in turn, to the transistor P2 in a current mirror circuit. The transistor P2 is also included in a differential amplifier stage that also comprises the transistor P3 and the constant current source I1. Here, the constant current source I1 is connected to the drain/source paths of the transistors P2 and P3. The transistors P3 and P4 form a further current mirror. The transistor P4 generates the current Iein that is coupled into the BGR circuit 1 from the setting circuit 2.
The function of the circuit arrangement as shown in FIG. 1 is as follows. The setting circuit 2 may be used to replicate in the transistor N1 the current flowing through the resistor R3 and the diode D1. For this purpose, the transistors N1 and N2 are set via their W/L ratio such that their steepness gm corresponds to the resistor R3. However, the resistor R3 and the steepness gm do not match because of fluctuations in the production process and different temperature coefficients. By contrast, the diode D1 has a similar temperature response and current response to those of the thermal voltage VT of the transistors N1 and N2. The arrangement shown in FIG. 1 thus yields only an inaccurate replication of the current flowing in the BGR cell 3 through the resistor R3 and the diode D1.
The current flowing through the transistor N1 is mirrored into the differential amplifier stage by means of the current mirror circuits constructed from the transistors N1 and N2 and, respectively P1 and P2. The current generated in the differential amplifier stage by the constant current source I1 is the minimum current that must flow through the transistor N1. If the current flowing through the transistor N1 is smaller than this minimum current, the differential amplifier stage causes the differential current of these two currents to flow through the drain/source path of the transistor P3. The current Iein is yielded as mirror image of the differential current by means of the current mirror constructed from the transistors P3 and P4.
The current Iein is coupled into the BGR circuit 1 at the non-inverting input of the operational amplifier OP1 and flows away there to ground VSS via the diode D1. As a result, the current Iein generates via the diode D1 a voltage drop that results, in turn, in a positive potential difference between the inputs of the operational amplifier OP1. The operational amplifier OP1 increases its output voltage because of the positive potential difference at its inputs.
The setting circuit 2 is designed such that the current Iein is switched off as soon as there is enough current flowing in the BGR cell 3 that it is possible to reach only the stable operating point of the BGR circuit 1. The current generated by the constant current source I1 in this case prescribes when the current Iein is switched off. The constant current source I1 can be constructed, for example from a resistor and a diode, or from a PTAT (Proportional to Absolute Temperature) generator.